Journal American Rhododendron Society

Research reveals successful method of rhododendron bud production
Harry H. Fries, County Agricultural Agent, Bridgeton, Cumberland County, N. J
Dr. James W. Paterson, Rutgers Research and Development Center, Bridgeton. N. J.
Reprinted with permission from the American Nurseryman

Discussions of setting rhododendron buds are never-ending among nurserymen. "High phosphorus," dry them down", "keep them hungry", "shock them" and other suggestions have been made, with everyone having his own set of practices to ensure a heavy bud set.
A replicated research trial, using 432 plants of each of three varieties ('Roseum Elegans', 'Chionoides' and 'Nova Zembla'), for a total of almost 1,300 plants, was set up to look at the effect of mulches, fertilizer and drip irrigation on rhododendron production. The trial was run at the Rutgers Research and Development Center, Bridgeton, N.J., in the middle of the $2 million rhododendron and broadleaved evergreen production area.
Treatments included bare ground, aluminum foil, clear plastic and black plastic, with fertilizer levels in each mulch of zero, 100 and 200 parts per million of nitrogen from 24-12-12 soluble fertilizer the first year and zero and 200 ppm of nitrogen from 20-2020 and 200 ppm of nitrogen from 2153-20 in the study's second year.
The loam/sandy loam soil had a 6.1 pH and soil test readings of very high phosphorus and high potassium at the start of the trial. Fifty pounds of nitrogen per acre were applied before mulching and planting.Mulches
The mulches were laid to provide a 4½-foot-wide bed, and the plants were spaced on 1½-foot centers and planted in late May. Beginning July 8, fertilizer was applied every two weeks through the drip irrigation system until mid October, except during August, when no fertilizer was applied.
The plants were grown for two years, but both the clear plastic and aluminum foil mulch disintegrated and were removed in early spring of the second year. The black mulch remained intact for the entire test.
Height measurements (two-way average) and number of breaks or branching were taken in October of the first year. Bush height and diameter and number of breaks and flower buds formed were determined in November of the second year and statistically analyzed.
The plants were graded in the spring (two years after planting) by a commercial nurseryman, and the commercial grades recorded and analyzed according to treatment. There were some interesting results:
• Fertilizer use increased plant size by five to 10 per cent, branching by 15 per cent and number of flower buds by 80 per cent.
• Black plastic mulch increased height by 30 percent, diameter by 65 per cent, branching by 110 per cent and number of flower buds by 115 per cent.
• On bare ground, all the plants graded out less than 12 inches. On black plastic mulch, plants graded as follows: 40 per cent was less than 12 inches; 30 per cent was 12 to 15 inches; 28 per cent was 15 to 18 inches, and two per cent was 18 inches or more.
• Winterkill and disease loss (Phytophthora cinnamomi) were more severe in fertilized plots (five per cent) than in unfertilized plots (two per cent). Mulching reduced winterkill and disease loss (2.8 per cent) better than bare ground (11.4 per cent).
Using disease-free plants and Truban drenches could reduce the disease loss in well-fertilized culture to maximize the benefits from the fertilizer.

In the picture at the left, the effects of two years of fertilizing three varieties of un-mulched rhododendrons
are shown. The stakes indicate missing plants due to disease and injury. Poor growth is noted on the
remaining plants. At the right, the effects of two years of fertilizing three varieties of rhododendrons
mulched with black plastic are shown. There are no missing plants and they exhibit good growth.

Bud Increases
This trial indicated that fertilizer and mulching increased branching of rhododendrons and the percentage of branches that bud up. (About 45 per cent budded with no fertilizer - 70 per cent budded with fertilizer.) The fertilizer was applied from late June to October in the second year of the study, with none applied during August. It was applied every two weeks during this period.
A high phosphorus fertilizer (21-5320) was compared with a 20-20-20. There was no significant difference in bud set, but a slight (10 percent) increase with the 21-53-20 was apparent, though not reliably reproducible.
Black plastic mulch may be a better investment than peat moss for rhododendron production. Commercially, peat moss is incorporated into the beds prior to planting. In this trial, no peat moss was used, which may explain the poor performance of the un-mulched plants.
However, the fact that rhododendrons require cool, well-drained, well-aerated soil would suggest that black plastic mulch would be ideal for them. Under black mulch, the soil pores remain open and the soil friable, even for two years, and the shading keeps the soil (two to four-inch depth) about 8° to 10° cooler during the summer. An added bonus is the positive weed control provided by the black mulch.
Drip irrigation is also a natural to combine with the mulch because it can be used to fertilize, water, apply fungicides and nematocides right in the root area with none applied in the alleys or vacant areas.

Per Plant Costs
Four-mil black plastic mulch film costs about 2¢ a square foot, or about 5¢ per plant at 1½-foot on center spacing. The twin wall drip tube, with two per bed, would cost about 4¢ per plant. The increase in value per plant from the mulch and drip tube would be about $1.20.
There are some other very big advantages with mulch and drip irrigation that are hard to pin down as far as costs are concerned. These include the weed control, potential to set up the irrigation on an automatic moisture controller to provide even moisture all season and the fact that drip irrigation takes a very low volume of water (smaller well and pump or more acreage from the same watering system).
Plastic mulch laying machines are available and can be modified to take a six-foot-wide roll of film to give a 4½ foot-wide mulched bed. Planting may take slightly longer than on bare ground but will be more precise. The drip irrigation system takes time to set up, but, once set, there are no pipes to move nor any delays when the plants need water.
The trial produced some interesting results and an indication of how to increase bud set and plant size with mulches and fertilizer. If nothing else, it should give the grower confidence to fertilize rhododendrons more than he has in the past, particularly in September and October. It may also encourage the grower to look into the use of black plastic mulch and drip irrigation for rhododendron production if he is interested in increasing the quality and turnover of this important crop.